Engineers at University of California, Berkeley, have built the first dust-sized, wireless sensors that can be implanted in the body, to monitor internal nerves, muscles or organs in real time, or to stimulate the immune system or tamp down inflammation.

Already shrunk to a 1 millimeter cube, about the size of a large grain of sand, and implanted in the muscles and peripheral nerves of rats, the so-called neural dust uses ultrasound both to power and read out the measurements.

It contains a piezoelectric crystal that converts ultrasound vibrations from outside the body into electricity to power a tiny, on-board transistor that is in contact with a nerve or muscle fiber.

A voltage spike in the fiber alters the circuit and the vibration of the crystal, which changes the echo detected by the ultrasound receiver, typically the same device that generates the vibrations, according to a release from UC Berkeley. The slight change, called backscatter, allows them to determine the voltage.

"I think the long-term prospects for neural dust are not only within nerves and the brain, but much broader," Michel Maharbiz, an associate professor of electrical engineering and computer sciences and one of the two main authors of a study published Wednesday in the journal Neuron, was quoted as saying.

"Having access to in-body telemetry has never been possible because there has been no way to put something supertiny superdeep. But now I can take a speck of nothing and park it next to a nerve or organ, your GI tract or a muscle, and read out the data."

In their experiment, the researchers powered up the passive sensors every 100 microseconds with six 540-nanosecond ultrasound pulses, to give them a continual, real-time readout.

They coated the first-generation motes -- 3 millimeters long, 1 millimeter high and 4/5 millimeter thick -- with surgical-grade epoxy, and they are currently building motes from biocompatible thin films which would potentially last in the body without degradation for a decade or more.

"The original goal of the neural dust project was to imagine the next generation of brain-machine interfaces, and to make it a viable clinical technology," said neuroscience graduate student Ryan Neely.

"If a paraplegic wants to control a computer or a robotic arm, you would just implant this electrode in the brain and it would last essentially a lifetime."

The team is working to miniaturize the device further, find more biocompatible materials and improve the surface transceiver that sends and receives the ultrasounds, ideally using beam-steering technology to focus the sounds waves on individual motes, and also building little backpacks for rats to hold the ultrasound transceiver that will record data from implanted motes.

"The beauty is that now, the sensors are small enough to have a good application in the peripheral nervous system, for bladder control or appetite suppression, for example," said Jose Carmena, a member of the Helen Wills Neuroscience Institute and another main author of the study.

"The technology is not really there yet to get to the 50-micron target size, which we would need for the brain and central nervous system. Once it's clinically proven, however, neural dust will just replace wire electrodes. This time, once you close up the brain, you're done."

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